DUV optical systems often employ DUV mirrors because refractive optical elements have a relatively low optical transmittance at DUV wavelengths (e.g., 190 nm to 280 nm). However, most optical materials used to form DUV mirrors do not have a sufficiently high reflectance at DUV wavelengths and thus require a reflectance coating.
A typical reflectance coating is formed from a stack of alternating layers of dielectric materials whose number, thickness and refractive index are selected to optimize reflectivity over a select wavelength band with a select center wavelength.
While the multilayer stack can be designed to provide a theoretical reflectance of 100%, variations in the fabricated film stack can act to reduce the reflectivity. These variations include surface roughness of the substrate, of the interface between the layers, and of the uppermost layer. Surface roughness causes light scattering, which in turn reduces the reflectance. This reduction in reflectance can be particularly problematic with DUV mirrors, which often require a reflectance of greater than 99%.